More than 1 million Web sites were believed to be infected with malware in the fourth quarter of last year, nearly double from the previous year, according to figures released today by Dasient.

Malvertising, advertising containing malware, also is on the rise, with impressions doubling to 3 million per day from the third quarter of 2010, Dasient said in a blog post.

The news corresponds with information released this week by another security firm. An analysis of than 3,000 Web sites across 400 organizations last year found that 44 percent of them had serious vulnerabilities at all times, while 24 percent were frequently vulnerable for an average of at least 270 days a year, according to WhiteHat Security, which provides Web site testing and security services for companies. Meanwhile, only 16 percent of the sites examined were found to be rarely vulnerable, the report said.